Novel cerium oxide particulates

ABSTRACT

Morphologically improved cerium oxide particulates having low sulfur content, useful, e.g., as catalyst/catalyst supports, and having a B.E.T. specific surface of at least 130 m 2  /g measured at a temperature ranging from 400° C. to 500° C., are prepared by reacting an aqueous solution of cerium (IV) salt with an aqueous solution of sulfate ions to precipitate a basic ceric sulfate, filtering the precipitate which results, washing and optionally drying said precipitate, and thence calcining same.

This application is a continuation of Ser. No. 07/000,779 filed Jan. 6,1987 now abandoned which in turn is a division of Ser. No. 06/703,474filed Feb. 20, 1985 now U.S. Pat. No. 4,661,330.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel ceric oxide having new andimproved morphological properties, and, more especially, a novel cericoxide having a low content in sulfur. It also relates to the preparationof such novel ceric oxide.

In the description which follows, by the term "specific surface" thereis intended the specific B.E.T. surface, determined in accordance withthe BRUNAUER-EMMETT-TELLER method described in Journal of the AmericanChemical Society, 60, 309 (1938).

2. Description of the Prior Art

It is known to this art that ceric oxide, whether alone or in a mixturewith other metallic oxides, is useful as a catalyst for the synthesis,in particular, of methanol [C. R. Seances, Acad. Sci., Ser. 2, 292 (12),883-5 (1981)] or in methods for treating residual gases (publishedJapanese Patent Application No. 76/62,616).

In order to demonstrate good catalytic reactivity, it is desirable touse a ceric oxide which has the greatest possible specific surface.

To date, however, most methods for the preparation of ceric oxide do notenable the attachment of this result.

For example, a method of preparing cerium (IV) oxide by the thermaldecomposition of cerium (III) oxalate is known, from the article by S.Horsely, J. M. Towner and M. B. Waldron [Preprints, Symp. Eur. Metall.Poudres, 4th, 1, paper 12 (1975)].

The subject treatment, carried out at 450° C., provides a ceric oxidehaving a specific surface of only 69 m² /g.

Compare also R. Sh. Mikhail, R. M. Cagr and R. B. Fahin, J. Appln.Chem., 20, 7, 222-225 (1970) which features the structure of cericoxide, and reports the characteristics of the ceric oxide prepared bythe calcining of ceric hydroxide obtained by treatment of a solution ofcerous nitrate with ammonia in the presence of hydrogen peroxide. It isnoted, however, that the ceric oxide obtained by calcining at 400° C.has a specific surface of only 80 m² /g.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofa novel ceric oxide having a specific surface greater than that hithertoknown to this art, i.e., a specific surface of at least 85±5 m² /g for atemperature of measurement ranging from 350° C. to 500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are photomicrographs taken with a scanning electronicmicroscope illustrating the morphology of the ceric oxide which can beobtained in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

More particularly according to the present invention, the novel cericoxide preferably has a specific surface of at least 130 m² /g at atemperature of from 400° C. to 500° C., and a maximum specific surfaceof about 150 to 180 m² /g at a temperature of from 400° C. to 500° C.

The ceric oxide according to the invention is also characterized in thatit comprises but a small amount of sulfur, present in the form ofsulfate ions. The amount of residual sulfate ions in the subject cericoxide will hereinafter be more fully described.

The invention also features a process for the preparation of the subjectceric oxide particulates having large specific surface, comprisingprecipitating basic ceric sulfate by reacting an aqueous solution ofcerium (IV) salt and an aqueous solution containing sulfate ions,filtering the resultant precipitate, washing and optionally drying same,and then calcining said washed and optionally dried precipitate.

In the first step of the process of the invention, a basic ceric sulfatehaving the formula Ce(OH)_(4-x) (SO₄)_(x/2) in which x is greater than Oand less than 4 is prepared.

For this purpose, one begins with an aqueous solution of cerium (IV)which may be an aqueous solution of ceric nitrate or an aqueous solutionof ammonium ceric nitrate. Said solution may contain cerium in cerousstate without any drawback, but it is desirable for it to contain atleast 85% of cerium (IV) in order to obtain a good yield ofprecipitation.

The cerium salt is selected in such manner that it does not containimpurities which could be transferred into the final product aftercalcining. It may be advantageous to use a cerium salt having a degreeof purity of more than 99%.

The concentration of the cerium salt solution is not a critical factoraccording to the invention. When expressed as cerium (IV), it may varyfrom 0.3 to 2 mols/liter.

The aqueous solution containing the sulfate ions is prepared from analkali or alkaline earth metal or ammonium sulfate, or by the bubblingof a gas containing sulfur in water, such as, for example, sulfurdioxide or sulfur trioxide. Ammonium sulfate is preferably used sincethe ammonium ion is easily eliminated upon the heat treatment of theprecipitate. It is preferable to use an ammonium sulfate having a degreeof purity of more than 99%.

Sulfuric acid can also be used a a generator of sulfate ions, providedthat the acidity imparted corresponds to, or is adjusted in such manneras to provide the conditions necessary for obtaining the desiredprecipitate.

The water comprising the aqueous solutions of cerium (IV) salt and thesolution containing the sulfate ions is preferably distilled water orwater purified by ion exchange.

The aqueous reaction medium advantageously has an acidity ranging from10⁻² N to 1 N, and preferably from 0.1 N to 0.5 N.

The acidity may be contributed by the commencement of a hydrolysisreaction, since one mole of hydrated ceric oxide is formed accompaniedby the liberation of 4 protons.

The acidity may also be imparted by the addition of an inorganic acidthereto. Preferably nitric acid or sulfuric acid is selected. An acid,which may be concentrated or dilute, for example, up to 10⁻² N, can beused.

The acidity may also emanate from the ceric nitrate solution, which maybe slightly acid and have a normality varying from 0.3 N to 5 N, andpreferably from 0.3 N to 1 N.

The amount of sulfate ions added is such that the ratio between themolar concentration of sulfate ions and the final equivalentconcentration of cerium (IV) ranges from 0.01 to 1.4 and preferably from0.3 to 0.4.

The equivalent final concentration of cerium (IV) is represented by theequation: ##EQU1## in which: ]Ce^(IV) ] is the concentration in molesper liter of the solution of cerium (IV) salt;

V is the volume of the solution of sulfate ions; and

V' is the volume of the solution of cerium (IV).

The proportions between the aqueous solution of cerium (IV) salt and theaqueous solution of sulfate ions is such that the equivalent finalconcentration of cerium (IV) ranges from 0.2 to 1.0 mole/liter, andpreferably from 0.3 to 0.6 mole/liter.

The precipitation of basic ceric sulfate, effected under thoseconditions above outlined, is preferably carried out at a temperature offrom 70° C. up to the reflux temperature of the reaction medium, whichis about 100° C.

It is easier to conduct the reaction at the reflux temperature as it isthus easy to control and reproduce.

In accordance with one first practical embodiment of the invention, theaqueous solution containing the sulfate ions is first heated until theselected desired temperature is attained within a predetermined range.

The solution of cerium (IV) salt is then introduced either fractionallyor continuously; the cerium (IV) salt solution is typically added over aperiod of time of from 1 to 4 hours. Compare the examples to follow forspecific illustration of suitable rates of addition of the cerium (IV)salt solution.

After completing the addition of said solution, the heating is continueduntil the cerium (IV) in the form of its basic sulfate has completelyprecipitated, whereupon the reaction mass is aged at this sametemperature before filtering. In this case, the aging time ranges from 2to 24 hours; the upper limit is not critical. However, a period of timewhich may range from 3 to 10 hours is typically satisfactory.

In accordance with a second embodiment of the first step of the processof the invention, the aqueous solution of the cerium (IV) salt and theaqueous solution containing the sulfate ions are mixed simultaneously atroom temperature and the resultant mixture is heated until attaining theselected desired temperature within the predetermined range.

Once this temperature has been reached, the heating is continued untilthe cerium (IV) in the form of its basic sulfate has completelyprecipitated, whereupon the reaction mass is aged under the conditionsdescribed above, typically for 2 to 24 hours and preferably from 3 to 10hours.

It should be pointed out that this embodiment is preferred since itmakes it possible to obtain a ceric oxide having the greatest porosity.

The second step of the process of the invention consists of filtering,after reaction, the reaction mass which is in the form of a suspension,the temperature of which ranges from 90° C. to 100° C. This operation iscarried out either before or after cooling the reaction mass to roomtemperature, namely, to a temperature ranging from about 10° C. to 25°C.

The filter cake is desirably washed to eliminate the nitrate ionsadsorbed on the precipitate.

Such washing is preferably carried out with distilled water or waterpurified by ion exchange, water whose temperature may vary, as desired,from 5° C. to 90° C. One or more washings is typically satisfactory,generally one to three washings.

The washing can also be carried out with an organic solvent. Exemplarysuch solvents are aliphatic, cycloaliphatic or aromatic hydrocarbons, oraliphatic or cycloaliphatic alcohols, such as methanol, ethanol,n-propanol, isopropanol, n-butanol, isobutanol and neobutanol. One ormore washings are carried out, typically one to three washings.

After washing, the water content of the filter cake ranges from 20% to80% and typically from 20% to 50%.

A preferred embodiment of the process of the invention comprisescarrying out an additional washing with a basic solution in order todisplace a portion of the sulfate ions.

An aqueous ammonia solution is preferably used for this purpose, theconcentration of which may vary, for example, from 0.1 N to 1 N.

Up to ten washings may be employed, but generally only one to fourwashings suffice.

The product obtained after filtration and successive washings is thendried in air or under reduced pressure on the order of 10⁻² to 100 mmmercury. The drying temperature advantageously varies from 90° C. to200° C.; the drying time is not critical and may range from 10 to 48hours.

In the final step of the process of the invention, the dried product iscalcined at a temperature which is preferably selected as the averagetemperature of intended use of the ceric oxide as catalyst.

The calcining temperature advantageously ranges from 300° C. to 500° C.,and preferably from 350° C. to 450° C. The time of calcination typicallyranges from about 30 minutes to ten hours.

The lower limit of the temperature range is not critical and may belowered.

On the other hand, there is no advantage in increasing the upper limitof the calcining temperature, since a decrease in the specific surfaceof the product ceric oxide is noted. Furthermore, it is pointed out thatthe product ceric oxide has a maximum specific surface on the order of150 to 180 m² /g after calcining at a temperature of 400° C. to 450° C.After calcination, the ceric oxide is recovered in very good yieldsince, when expressed as cerium (IV), it represents 80% to 98% of thecerium (IV) present in the initial solution of cerium (IV) salt.

It will also be appreciated that the process of the invention isextremely well adopted to be carried out continuously.

The process of the invention can be carried out in conventionalapparatus. The step of the precipitation of the basic ceric sulfate iscarried out in a reactor equipped with a thermo-regulated heatingdevice, customary reaction control means (thermometer), agitating means(anchor or propeller agitation), and means for the introduction of thereagents.

Aging can advantageously be carried out in the reactor or in anotherreactor of the same type. The transfer of the reaction mass from onereactor to the other may be effected by gravity or by means of pumps.

The filtration of the resultant suspension can then be effected on afilter under the pressure of an inert gas such as nitrogen, on a filterunder reduced pressure (Buchner, Nutche), or else on a continuousfiltration device, for example, a rotary filter of the Vernay type or aband filter.

The precipitate is then placed in silica, porcelain or alumina boats andis then subjected to the drying operation, which may be carried out inany drying device, for example, in a stove which is either vented ormaintained under reduced pressure.

It is then subjected to calcination, for example, in a chamber furnace,tunnel, muffle oven or rotary furnace provided with means enablingregulation of the temperature during the heat treatment.

By the process of the invention, a ceric oxide is produced whichcontains a certain amount of sulfate ions, the presence of which affectsthe specific surface obtained.

The amount of residual sulfate ions depends upon the amount of sulfateions introduced during the precipitation reaction and on the fact thatsaid ions have been eliminated to a greater or lesser extent during thewashing operations, in particular during the washing with ammonia.

The Table I which follows sets forth, by way of illustration, theamounts of residual sulfate ions and the specific surfaces obtained,depending upon whether or not the precipitate was washed with ammoniaand depending upon the number of washings with ammonia, in all cases theamount of sulfate ions added during the course of the precipitation,expressed by the molar ratio [SO₄ ⁼ ]/[Ce^(IV) eq], being 0.36 and theprecipitate being washed once with water and then calcined for six hoursat 400° C.

The residual amount of sulfate ions in the product ceric oxide isexpressed by the molar ratio [SO₄ ⁼ ]/[Ce^(IV) ].

                  TABLE I                                                         ______________________________________                                                     Amount of specific                                                            residual sulfate                                                                        surface                                                             ions      of CeO.sub.2                                           ______________________________________                                        Without washing                                                                              0.30         60 to 120 m.sup.2 /g                              with ammonia                                                                  With one washing                                                                             0.09        120 to 160 m.sup.2 /g                              with 0.4 N ammonia                                                            With four washings                                                                           0.0018      150 to 170 m.sup.2 /g                              with 0.4 N ammonia                                                            With ten washings                                                                            0.0009      150 to 180 m.sup.2 /g                              with 0.4 N ammonia                                                            ______________________________________                                    

For catalytic uses of the product ceric oxide it is desirable that theamount of residual sulfate ions be such that the aforesaid ratio is lessthan 0.03. It is preferably less than 0.002.

Even more preferably, it is advisable that it be as close as possible tozero.

FIGS. 1 and 2 are photomicrographs taken with a scanning electronicmicroscope (enlargement: 300 and 12,000) which evidence the morphologyof the ceric oxide obtained in accordance with the invention.

It will be noted that the particles of ceric oxide have pore dimensionsof an average diameter of about 30 Å.

The uses to which the ceric oxide prepared in accordance with theinvention can be put are very numerous. Mention may be made, inparticular, of applications as filler, binder, wash coat, thickener,dispersant, reinforcer, pigment, adsorbent and raw material for themanufacture of ceramics and of glass polishing compositions.

The ceric oxide of the invention has an unexpectedly great surface, suchthat it is eminently well suited for use in the field of catalysis,either as catlayst, per se, or catalyst support.

The ceric oxide particulates of the invention may be employed ascatalyst or catalyst support to carry out a wide variety of reactionssuch as, for example, dehydration, hydrosulfuration,hydrodenitrification, desulfurization, hydrodesulfurization,dehydrohalogenation, reforming, steam reforming, cracking,hydrocracking, hydrogenation, dehydrogenation, isomerization,dismutation, oxychlorination, dehydrocyclization of hydrocarbons orother organic compounds, oxidation and/or reduction reactions, the Clausreaction, the treatment of exhaust gases from internal combustionengines, demetallization, methanation, and shift conversion.

Too, the ceric oxide of the invention can be used either alone or inadmixture with a wide variety of other oxides.

Due to its great chemical reactivity, it can also be advantageously usedfor the production of mixed catalyst supports, for example, Al₂ O₃--MgO--CeO₂ (see published Japanese Patent Application No. 78/40,077).

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

Into a 6-liter three-neck round-bottom flask provided with athermometer, an agitator, reagent inlet means (metering pump) and areflux condenser, and equipped with a heating device, there wereintroduced, at room temperature, in any order, 1909 cc of water purifiedby ion exchange and containing 37.4 g of ammonium sulfate (NH₄)SO₄ and610 cc of a ceric nitrate solution containing 1.275 moles/liter ofcerium (IV), 0.025 moles/liter of cerium (III) and having a free acidityof 0.365 N.

A mixture having an equivalent final cerium (IV) concentration of 0.465moles/liter was obtained.

Said mixture was heated to the boiling point thereof and then refluxedfor 24 hours.

Filtration of the resulting precipitate was carried out over frittedglass (porosity No. 3) and three washings with water were effected at90° C.

The cerium (IV) contained in the mother liquors was determined bypotentiometric titration, using a titrated solution of iron (II), whichenabled determination of a precipitation yield of 96.3%.

Before calcining, four successive washings were carried out using an 0.4N ammonia solution at 80° C.

The product obtained was next dried at 105° C. for 48 hours and thencalcined in a muffle furnace for 6 hours at 400° C.

A ceric oxide was produced, containing sulfate ion in an amount suchthat the molar ratio [SO₄ ⁼ ]/[Ce^(IV) ] was equal to 0.002.

It had a specific surface of 164 m² /g.

The product ceric oxide was of fluorine-type structure, namely, facecentered cubic.

The parameter and the intensity of the CaF₂ structure were as follows:

    ______________________________________                                        (i)    Lattice parameter                                                                              a = 5.435 ± 0.01Å                              (ii)   Crystallization rate                                                                           t = 55%.                                              ______________________________________                                    

The lattice parameter of pure ceric oxide is 5.411 Å (JCPDS 4 0593).

It is therefore noted that the lattice parameter was slightly expanded.

EXAMPLE 2

This example demonstrates the influence of the washing with an ammoniasolution on the specific surface of the product ceric oxide.

For this purpose, Example 1 is repeated, with the sole difference beingthat the washing with ammonia was eliminated.

The calcining was carried out under the same conditions.

A ceric oxide containing sulfate in an amount such that the molar ratio[SO₄ ⁼ ]/[Ce^(IV) ] was equal to 0.26 was obtained.

It had a specific surface of 110 m² /g.

EXAMPLES 3 TO 5

These examples demonstrate the influence of the concentration of the SO₄⁼ in the reaction medium on the specific surface of the product cericoxide.

Example 1 was repeated, except that the amount of ammonium sulfateintroduced into the reaction medium was varied.

The following Table II reports the specific surface of the ceric oxideobtained after calcining at 400° C. for 6 hours as a function of the SO₄⁼ content of the reaction medium. For purposes of comparison, theresults obtained in Example 1 have been included.

                  TABLE II                                                        ______________________________________                                        No. of    Amount of ammonium                                                                           Specific surface of                                  Example   sulfate introduced                                                                           CeO.sub.2                                            ______________________________________                                        3         6.9       g        80     m.sup.2 /g                                1         37.4      g        164    m.sup.2 /g                                4         69.11     g        122    m.sup.2 /g                                5         137       g        78     m.sup.2 /g                                ______________________________________                                    

EXAMPLE 6

In this example, a solution of ceric nitrate and an aqueous solutioncontaining sulfate ions, identical to those described in Example 1, wereused.

The precipitation of the basic sulfate was carried out by adding theceric nitrate solution, at a rate of flow of 203.3 cc/hour, to theaqueous solution containing the sulfate ions which had previously beenadjusted to reflux. Upon completion of the addition to said solution,the reaction mass was aged for three hours at the reflux temperature.

Filtering, drying and calcining identical to those described in Example1 were then carried out on the basic sulfate obtained.

The ceric oxide thus produced had a high specific surface, on the orderof 160 m² /g, but a smaller pore volume than the ceric oxide prepared asin Example 1.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A ceric oxide composition comprising ceric oxideparticulates, said particulates consisting essentially of ceric oxide,said particulates having a B.E.T. specific surface of at least 130 m²/g, measured at a temperature ranging from 400° C. to 500° C. and saidparticulates having a content in residual sulfate ions, expressed as themolar ratio [SO₄ ⁼ ]/]Ce^(IV) ], of less than 0.03.
 2. Ceric oxideparticulates consisting essentially of ceric oxide and having a B.E.T.specific surface of at least 130 m² /g, measured at a temperatureranging from 400° to 500° C.
 3. The ceric oxide particulates as definedby claim 2, having a photomicrograph thereof comprising those of FIGS. 1or
 2. 4. Ceric oxide particulates according to claim 2 having a contentin residual sulfate ions, expressed as the molar ratio [SO₄ ⁼ ]/[Ce^(IV)], of less than 0.03.
 5. The ceric oxide particulates as defined byclaim 4, having a B.E.T. specific surface of from 150 to 180 m² /g,measured at a temperature ranging from 400° C. to 500° C.
 6. The cericoxide particulates as defined by claim 4, having a content in residualsulfate ions, expressed as the molar ratio [SO₄ ⁼ ]/[Ce^(IV) ], of lessthan 0.002.
 7. The ceric oxide particulates as defined by claim 4,having an average pore diameters of about 30 Å.